Defective pixel correction device

ABSTRACT

First pixel data of a pixel of interest is output from a first shift register, while second and third pixel data of neighboring pixels indicative of the same color are output from second and third shift registers, respectively. Differential data between estimated pixel data calculated from the second and third pixel data and the first pixel data is input to a comparator. A threshold value stored in a register is modulated by the estimated pixel data, and is input to the comparator as modulated threshold data. When the comparator judges that the differential data is greater than the modulated threshold data, a selector outputs the estimated pixel data as corrected pixel data.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of and claims the benefit of priorityunder 35 U.S.C. §120 from U.S. Ser. No. 10/814,256, filed Apr. 1, 2004,and claims the benefit of priority under 35 U.S.C. §119 from JapanesePatent Application priority document 2003-107851 filed in Japan on Apr.11, 2003.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a technique of correcting defectivepixels present in a sensor.

2. Description of the Background Art

Sensors such as CCDs (Charge Coupled Devices) may contain defectivepixels containing white spot defects, black spot defects or the like.Such defective pixels affect image quality, and thus require correction.For instance, CCD manufacturing plants employ a method of identifyingaddresses of such defective pixels and storing information on theaddresses in a nonvolatile memory, so that CCDs are shipped with suchinformation. Accordingly, when capturing images, digital cameras and thelike use a method of identifying the position of a defective pixel basedon such address information to correct the defective pixel by itsneighboring pixels. This method requires a memory for storing theaddress information, which interferes with size reduction in circuitscale.

In this respect, Japanese Patent Application Laid-Open No. 2002-223391discloses a technique of eliminating the need to provide a memory forstoring address information.

On the other hand, Japanese Patent Application Laid-Open No. 2002-142157discloses a technique of correcting defective pixels in accordance withthe brightness level of images.

However, the conventional technique of eliminating the need to provide amemory for storing address information is disadvantageous in accuracy ofcorrection, while the conventional technique of correcting defectivepixels in accordance with the brightness level of images causes increasein circuit scale.

SUMMARY OF THE INVENTION

The present invention is directed to a defective pixel correctiondevice.

According to an aspect of the present invention, the defective pixelcorrection device comprises: a circuit for receiving image data from asensor; a circuit for obtaining an estimated pixel value of a pixel ofinterest from pixel data of neighboring pixels of the pixel of interest;a circuit for obtaining a differential value between a sensor-inputpixel value of the pixel of interest and the estimated pixel value; amodulation circuit for modulating a predetermined threshold value by theestimated pixel value, thereby obtaining a modulated threshold value;and a circuit for comparing the differential value and the modulatedthreshold value, and when the differential value is greater than themodulated threshold value, outputting the estimated pixel value as apixel value of the pixel of interest instead of the sensor-input pixelvalue.

According to the present invention, defective pixel correctionappropriate to brightness can be performed. Further, there is no need toprovide a memory for storing address information on a defective pixel,enabling reduction in circuit scale.

According to another aspect of the present invention, the defectivepixel correction device comprises: a circuit for receiving pixel datafrom a color sensor; a circuit for obtaining an estimated pixel value ofa pixel of interest from pixel data of neighboring pixels of the pixelof interest indicative of the same color as the pixel of interest; acircuit for obtaining a differential value between a sensor-input pixelvalue of the pixel of interest and the estimated pixel value; a circuitfor storing a threshold value corresponding to each color of the colorsensor; and a comparing and selecting circuit for comparing thedifferential value and the threshold value corresponding to the color ofthe pixel of interest, and when the differential value is greater thanthe threshold value, outputting the estimated pixel value as a pixelvalue of the pixel of interest instead of the sensor-input pixel value.

According to the present invention, a threshold value is prepared foreach color, enabling defective pixel detection with higher accuracy.

It is therefore an object of the present invention to provide adefective pixel correction technique capable of performing appropriatecorrection with high accuracy while achieving reduction in circuitscale.

These and other objects, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a digital camera;

FIG. 2 is a functional block diagram of a defective pixel correctioncircuit according to a first preferred embodiment of the presentinvention;

FIG. 3 is a functional block diagram of a converter for modulatingthreshold values according to the first preferred embodiment;

FIG. 4 shows the relationship between brightness and modulationcoefficient;

FIG. 5 is a functional block diagram of a defective pixel correctioncircuit according to a second preferred embodiment of the invention;

FIG. 6 is a functional block diagram of a defective pixel correctioncircuit according to a third preferred embodiment of the invention;

FIG. 7 is a functional block diagram of a converter for modulatingthreshold values according to the third preferred embodiment; and

FIG. 8 shows the relationship between brightness and modulationcoefficient.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be described inreference to the accompanying drawings.

First Preferred Embodiment

FIG. 1 is a schematic block diagram of a digital camera 8 including adefective pixel correction circuit 1 according to a first preferredembodiment of the present invention. In the digital camera 8, reflectedlight from a subject is received by a CCD imager 5 through an opticalsystem 4. The CCD imager 5 according to the present embodiment is acolor sensor equipped with Bayer pattern three color filters of R (red),G (green) and B (blue). Analog signals indicative of respective RGBcolors output from the CCD imager 5 undergo various kinds of analogprocessing, and are thereafter converted into digital form in an A/Dconverter 6, to be input to an image processing section 7 as digitalimage data. In the present embodiment, as shown in the diagram, pixeldata is output in pixel lines in the order of RGRG and GBGB alternatelyand repetitively. This is merely an example, and the order of colorsbeing output is different depending on the color filter pattern of theCCD imager 5.

The image processing section 7 is a functional section for executingvarious kinds of digital processing on input pixel data. The defectivepixel correction circuit 1 corrects pixel data derived from a defectivepixel present in the CCD imager 5.

FIG. 2 is a functional block diagram of the defective pixel correctioncircuit 1. Pixel data 70 output from the CCD imager 5 is input to thedefective pixel correction circuit 1, and specifically, to a shiftregister including registers 11 to 15.

The registers 11 to 15 are each capable of holding bit datacorresponding to one pixel, and each transfer pixel data correspondingto one pixel which they are holding to a subsequent register insynchronization with a clock signal 80. Therefore, the registers 11 to15 can hold pixel data corresponding to five pixels. As shown in thediagram, respective pieces of pixel data output from the registers 11 to15 are indicated by 71 to 75.

Since the pixel data 70 input to the defective pixel correction circuit1 represents a line of pixels in the order of RGRG or BGBG as describedabove, pixel data indicative of one of RGB is stored in alternate onesamong the registers 11 to 15. Therefore, the registers 11, 13 and 15each hold pixel data indicative of the same color, and the respectivepieces of pixel data 71, 73 and 75 output in synchronization with theclock signal 80 are indicative of the same color.

The pieces of pixel data 71 and 75 output at a certain clock cycle areadded in an adder 21. Output data 76 from the adder 21 is divided by twoin an average calculating circuit 22. Estimated pixel data 77 is therebyoutput.

Here, the respective pieces of pixel data 71 and 75 are indicative ofthe same color, and are located ahead and behind the pixel data 73,respectively. Assuming that the pixel data 73 is derived from a pixel ofinterest, the estimated pixel data 77 indicates the average of pixelvalues of two pixels of the same color located ahead and behind thepixel of interest. In the case where the pixel of interest is adefective pixel, the pixel data 77 is used for estimating the pixelvalue of the pixel of interest instead of the pixel data 73. That is,the pixel data 77 is data obtained from neighboring pixels of the pixelof interest for estimating the pixel value of the pixel of interest.

Next, differential data 78 indicative of the difference between theestimated pixel data 77 and pixel data 73 is calculated in a subtracter23. The pixel data 73 indicates the pixel value of the pixel of interestinput from the CCD imager 5. In other words, the pixel data 73 indicatesa sensor-input pixel value while the estimated pixel data 77 indicatesan estimated pixel value of the pixel of interest.

Further, an absolute value arithmetic circuit 24 obtains the absolutevalue of the differential data 78, so that differential data 79 isoutput. The differential data 79 is input to a comparator 27.

On the other hand, the estimated pixel data 77 output from the averagecalculating circuit 22 is also input to a converter 26.

FIG. 3 is a functional block diagram of the converter 26. The estimatedpixel data 77 input to the converter 26 is then input to a coefficientcalculating section 261, so that a coefficient for threshold modulationis calculated. The process of calculating the coefficient from theestimated pixel data 77 is executed using the function shown in thegraph of FIG. 4. In the graph, the horizontal axis indicates the valueof the estimated pixel data 77 (i.e., pixel value), and the verticalaxis indicates the coefficient. Here, the estimated pixel data 77 is10-bit data and is converted to a 10-bit coefficient using the function,by way of example.

As shown in the graph, the function for obtaining the coefficient is nota simple linear function, but is set to have a different inclination ineach range of pixel values of the estimated pixel data 77. That is, adifferent function is applied to each brightness level. The inclinationsof the function as shown in the graph are just an example, and anyfunction can be set in each range.

However, as shown in FIG. 4, it is preferable to set the function suchthat the coefficient increases as the pixel value of the estimated pixeldata 77 increases, that is, as brightness increases. This can suppressover-correction in a high-brightness area, and can also prevent failurein correction in a middle-brightness area and a dark area.

Coefficient data 265 output from the coefficient calculating section 261is multiplied by threshold data 90 output from a register 25 (shown inFIG. 2) in a multiplier 262, so that modulated threshold data 266 isobtained. The threshold data 90 is data indicative of a threshold valuefor determining a defective pixel. Modulating the threshold data 90 bythe coefficient data 265 causes the threshold value to be modulated inaccordance with the brightness of the estimated pixel data 77. Themodulated threshold data 266 thereby obtained is divided by 128 in adivider 263.

In the present example, the threshold data 90 is multiplied by thecoefficient data 265 indicative of 0 to 1023 (in 10 bits) in themultiplier 262 for modulation, and is divided by 128 (7-bit shiftoperation), which means the threshold value is modulated in eight waysof 0 to 7 times. The converter 26 thereby outputs modulated thresholddata 91. Adjusting a divisor used in the divider 263 allows the level ofthreshold modulation to be arbitrarily changed.

Referring back to FIG. 2, the modulated threshold data 91 output fromthe converter 26 is input to the comparator 27. The comparator 27compares the differential data 79 and modulated threshold data 91 interms of size, and outputs a selection signal 81 according to therelationship in terms of size. Specifically, when the differential data79 is smaller than the modulated threshold data 91, “0” is output as theselection signal 81. When the differential data 79 is greater than themodulated threshold data 91, “1” is output as the selection signal 81.

The selection signal 81 is input to a selector 28 as well as the pixeldata 73 and estimated pixel data 77. When the selection signal 81indicates “0”, the selector 28 outputs the pixel data 73 as correctedpixel data 92, that is, employs a sensor-input pixel value as the pixeldata of the pixel of interest. When the selection signal 81 indicates“1”, the selector 28 outputs the estimated pixel data 77 as thecorrected pixel data 92, that is, employs an estimated pixel value asthe pixel data of the pixel of interest in place of a sensor-input pixelvalue. In this way, the selector 28 outputs the corrected pixel data 92in response to respective pieces of pixel data 70 output from the CCDimager 5.

The defective pixel correction circuit 1 according to the presentembodiment outputs image data captured by the digital camera 8 afterperforming real-time processing and defective pixel correction, whichtherefore eliminates the need to provide a memory for storing addressesof defective pixels. Further, it is unnecessary to capture referencelevels such as black level and white level at start up of equipment,allowing starting-time to be shortened.

Furthermore, the threshold value for determining a defective pixel ismodulated in accordance with the value of the estimated pixel data 77(i.e., pixel value), which achieves improved accuracy in determiningdefective pixels. Specifically, it is possible to preventover-correction occurring in the case where a defective pixel isdetermined on the basis of comparison with a fixed threshold value aswell as to correct a defective pixel in a middle-brightness area or darkarea which will not be determined as a defective pixel using a fixedthreshold value.

In the present embodiment, estimated pixel data is generated using twopixels ahead and behind a pixel of interest, however, neighboring pixelsto be chosen are not limited thereto. For instance, the estimated pixeldata may be generated using four neighboring pixels on top, bottom,right and left of a pixel of interest on a two-dimensional image.

Alternatively, where the primary object lies in reducing circuit scale,one pixel may be chosen as a neighboring pixel, and the pixel value ofthat pixel may be employed as the estimated pixel value of a pixel ofinterest. In this case, referring to FIG. 2, the registers 11 and 12 areremoved, so that the registers 13, 14 and 15 constitutes a shiftregister. Then, the pixel data input to the register 15 is employed asthe estimated pixel data while the pixel data input to the register 13is derived from the pixel of interest. The pixel data stored in theregister 15 is already subjected to defective pixel correction, and isthus suitable for use as the estimated pixel data. Therefore, it is notnecessary to provide the adder 21 or average calculating circuit 22. Thesubtracter 23 calculates the difference between the pieces of pixel data73 and 75, to obtain the differential data 78.

Second Preferred Embodiment

A defective pixel correction circuit 2 according to a second preferredembodiment will be described now. The defective pixel correction circuit2 is also incorporated into an image pickup device or the like toperform correction of defective pixels. In the present embodiment, asshown in FIG. 1, the defective pixel correction circuit 2 isincorporated into the digital camera 8 for correcting pixel data outputfrom the CCD imager 5.

FIG. 5 is a block diagram of the defective pixel correction circuit 2 ofthe present embodiment. In the diagram, similar functional sections asthose described in the first preferred embodiment are indicated by thesame reference numerals. The following description will be focused onfunctional sections different from those of the first preferredembodiment.

The respective pieces of pixel data 71 and 75 output from the registers11 and 15 are processed in the adder 21 and average calculating circuit22, so that the estimated pixel data 77 is obtained. The differentialdata 79 obtained through the subtracter 23 and absolute value arithmeticcircuit 24 is input to the comparator 27. The estimated pixel data 77 isalso input to the converter 26.

Different from the first preferred embodiment, the defective pixelcorrection circuit 2 includes a plurality of registers 251 to 254 forstoring threshold values corresponding to the respective colors of pixeldata output from the CCD imager 5. The registers 251 and 252 store Rthreshold data 101 and G threshold data 102, respectively, incorrespondence with the pixel line in the order of RGRG. The registers253 and 254 store B threshold data 103 and G threshold data 104,respectively, in correspondence with the pixel line in the order ofBGBG. The two pieces of G threshold data 102 and 104 may be stored in acommon register.

A selection signal 100 is input to a selector 31 as well as thethreshold data 101 to 104 for the respective colors. The selectionsignal 100 is supplied from a timing generator not shown, foridentifying the color of the pixel data 71, 73 and 75 (all indicative ofthe same color) currently being output from the registers 11, 13 and 15,respectively. In the pixel line in the order of RGRG, the selectionsignal 100 indicates “0” when R is output, and “1” when G is output. Inthe pixel line in the order of BGBG, the selection signal 100 indicates“2” when B is output, and “3” when G is output.

In response to the selection signal 100, the selector 31 outputsthreshold data 105 for the corresponding color. The threshold data 105is input to the converter 26, where the same processing is conducted asin the first preferred embodiment, and modulated threshold data 106 isoutput.

The comparator 27 compares the differential data 79 and modulatedthreshold data 106 to output “0” as the selection signal 81 when thedifferential data 79 is smaller than the modulated threshold data 106and output “1” as the selection signal 81 when the differential data 79is greater than the modulated threshold data 106. Then, the selector 28outputs the pixel data 73 as corrected pixel data 92 when the selectionsignal 81 indicates “0”, and outputs the estimated pixel data 77 as thecorrected pixel data 92 when the selection signal 81 indicates “1”. Thecorrected pixel data 92 is thereby output in response to each piece ofpixel data 70 output from the CCD imager 5.

As described, according to the second preferred embodiment, the mostsuitable modulated threshold value can be set for each color, enablingdetection of defective pixels with higher accuracy.

In the present embodiment, estimated pixel data is generated using twopixels ahead and behind a pixel of interest, however, neighboring pixelsto be chosen are not limited thereto. For instance, the estimated pixeldata may be generated using four neighboring pixels on top, bottom,right and left of a pixel of interest on a two-dimensional image.Alternatively, where the primary object lies in reducing circuit scale,one pixel may be chosen as a neighboring pixel, and the pixel value ofthat pixel may be employed as the estimated pixel value of a pixel ofinterest.

Third Preferred Embodiment

A defective pixel correction circuit 3 according to a third preferredembodiment will be described now. The defective pixel correction circuit3 is also incorporated into an image pickup device or the like toperform correction of defective pixels. In the present embodiment, asshown in FIG. 1, the defective pixel correction circuit 3 isincorporated into the digital camera 8 for correcting pixel data outputfrom the CCD imager 5.

FIG. 6 is a block diagram of the defective pixel correction circuit 3 ofthe present embodiment. In the diagram, similar functional sections asthose described in the first preferred embodiment are indicated by thesame reference numerals. The following description will be focused onfunctional sections different from those of the first preferredembodiment.

The respective pieces of pixel data 71 and 75 output from the registers11 and 15 are processed in the adder 21 and average calculating circuit22, so that the estimated pixel data 77 is obtained. Further, thedifferential data 79 obtained through the subtracter 23 and absolutevalue arithmetic circuit 24 is output. Different from the firstpreferred embodiment, the differential data 79 is input to a converter41. The estimated pixel data 77 is also input to the converter 41.

FIG. 7 is a functional block diagram of the converter 41. The estimatedpixel data 77 input to the converter 41 is then input to a coefficientcalculating section 411, so that a coefficient for threshold valuemodulation is calculated. The process of calculating the coefficientfrom the estimated pixel data 77 is executed using the function shown inthe graph of FIG. 8. In the graph, the horizontal axis indicates thevalue of the estimated pixel data 77(i.e., pixel value), and thevertical axis indicates the coefficient. Here, the estimated pixel data77 is 10-bit data and is converted to a 10-bit coefficient using thefunction, by way of example.

As shown in the graph, the function for obtaining the coefficient is nota simple linear function, but is set to have a different inclination ineach range of pixel value of the estimated pixel data 77. That is, adifferent function is applied to each brightness level. The inclinationsof the function as shown in the graph are just an example, and anyfunction can be determined in each range.

However, as shown in FIG. 8, it is preferable to set the function suchthat the coefficient increases as the pixel value of the estimated pixeldata 77 decreases, that is, as brightness decreases. This can suppressover-correction in a high-brightness area, and can also prevent failurein correction in a middle-brightness area and a dark area.

Coefficient data 415 output from the coefficient calculating section 411is multiplied by differential data 79 in a multiplier 412, so thatmodulated differential data 416 is obtained. The modulated differentialdata 416 thereby obtained is divided by 128 in a divider 413. In thepresent example, the differential data 79 is multiplied by thecoefficient data 415 indicative of 0 to 1023 (in 10 bits) formodulation, and is divided by 128 (7-bit shift operation), which meansthe modulated differential value is modulated in eight ways of 0 to 7times. The converter 41 thereby outputs modulated differential data 110.

Referring back to FIG. 6, threshold data 111 output from a register 43and modulated differential data 110 output from the converter 41 areinput to a comparator 42. Here, the threshold data 111 is indicative ofa threshold value for determining a defective pixel, and is compared interms of size with the modulated differential data 110 modulated by thecoefficient data 415.

According to the relationship in terms of size, the comparator 42outputs the selection signal 81. Specifically, when the modulateddifferential data 110 is smaller than the threshold data 111, “0” isoutput as the selection signal 81. When the modulated differential data110 is greater than the threshold data 111, “1” is output as theselection signal 81.

When the selection signal 81 indicates “0”, the selector 28 outputs thepixel data 73 as the corrected pixel data 92, that is, employs asensor-input pixel value as the pixel data of a pixel of interest. Whenthe selection signal 81 indicates “1”, the selector 28 outputs theestimated pixel data 77 as the corrected pixel data 92, that is, employsan estimated pixel value as the pixel data of the pixel of interest. Inthis way, the selector 28 outputs the corrected pixel data 92 inresponse to respective pixel data 70 output from the CCD imager 5.

As described, image data captured by the digital camera 8 is outputafter undergoing real-time processing and defective pixel correction,which therefore eliminates the need to provide a memory for storingaddresses of defective pixels. Further, the differential value fordetermining a defective pixel is modulated in accordance with the valueof the estimated pixel data 77 (i.e., pixel value), which achievesimproved accuracy in determining defective pixels.

In the present embodiment, estimated pixel data is generated using twopixels ahead and behind a pixel of interest, however, neighboring pixelsto be chosen are not limited thereto. For instance, the estimated pixeldata may be generated using four neighboring pixels on top, bottom,right and left of a pixel of interest on a two-dimensional image.

Alternatively, where the primary object lies in reducing circuit scale,one pixel may be chosen as a neighboring pixel, and the pixel value ofthat pixel may be employed as the estimated pixel value of a pixel ofinterest. In this case, referring to FIG. 6, the registers 11 and 12 areremoved, so that the registers 13, 14 and 15 constitutes a shiftregister. Then, the pixel data input to the register 15 is employed asthe estimated pixel data while the pixel data input to the register 13is derived from the pixel of interest. The pixel data stored in theregister 15 is already subjected to defective pixel correction, and isthus suitable for use as the estimated pixel data. Therefore, it is notnecessary to provide the adder 21 or average calculating circuit 22. Thesubtracter 23 calculates the difference between the respective pieces ofpixel data 73 and 75, to obtain the differential data 78.

The construction according to the present embodiment may be combinedwith that of the second preferred embodiment. Specifically, the register43 for storing the threshold data 111 shown in FIG. 6 may be substitutedby the circuit including the four registers 251 to 254 and selector 31.This allows the most suitable modulated threshold value to be set foreach color, enabling detection of defective pixels with higher accuracy.

The above three preferred embodiments have described by way of examplethat the defective pixel correction circuits are each incorporated intothe digital camera, however, the defective pixel correction circuits ofthe respective preferred embodiments may be incorporated into varioustypes of image pickup devices and image readers such as digital moviesand image scanners.

Further, the above preferred embodiments have described by way ofexample that defective pixel correction is performed on pixel dataindicative of RGB, however, the present invention is also applicable tovarious image pickup devices in which pixel data indicative of Y(yellow), C (cyan) and M (magenta) is input from a CCD imager equippedwith CMY filters, and defective pixel correction is performed usingpixel data of these complementary colors.

Furthermore, pixel data to be processed is not limited to that outputfrom a CCD imager, but pixel data output from various types of sensorssuch as CMOS sensors may be adopted.

While the invention has been shown and described in detail, theforegoing description is in all aspects illustrative and notrestrictive. It is therefore understood that numerous modifications andvariations can be devised without departing from the scope of theinvention.

1. A defective pixel correction device, comprising: a first circuitconfigured to receive pixel data from a color sensor; a second circuitconfigured to obtain an estimated pixel value of a pixel of interestfrom pixel data of neighboring pixels of said pixel of interest having asame color as said pixel of interest; a difference circuit configured toobtain a differential value between a sensor-input pixel value of saidpixel of interest and said estimated pixel value; a storing circuitstoring a plurality of threshold values, one for each color of saidcolor sensor; and a comparing and selecting circuit configured tocompare said differential value and a continuously varying thresholdvalue of the plurality of threshold values stored in the storing circuitcorresponding to the color of said pixel of interest, and when saiddifferential value is greater than said threshold value, to output saidestimated pixel value as a pixel value of said pixel of interest insteadof said sensor-input pixel value.
 2. The defective pixel correctiondevice according to claim 1, further comprising: a modulation circuitconfigured to modulate said threshold value corresponding to the colorof said pixel of interest by said estimated pixel value, therebyobtaining a modulated threshold value, wherein said comparing andselecting circuit includes a comparing circuit configured to comparesaid differential value and said modulated threshold value, and whensaid differential value is greater than said modulated threshold value,to output said estimated pixel value as a pixel value of said pixel ofinterest instead of said sensor-input pixel value.
 3. The defectivepixel correction device according to claim 1, further comprising: amodulation circuit configured to modulate said differential value bysaid estimated pixel value, thereby obtaining a modulated differentialvalue, wherein said comparing and selecting circuit includes a comparingcircuit configured to compare said modulated differential value and saidthreshold value, and when said modulated differential value is greaterthan said threshold value, to output said estimated pixel value as apixel value of said pixel of interest instead of said sensor-input pixelvalue.
 4. The defective pixel correction device of claim 1, furthercomprising: a selector configured to select and output one of theplurality of stored threshold values as the threshold value that iscompared to the differential value, based on an input selection signal.